Terminate DocumentsUI opened for the specific root.

If USB is opened in DocumentsUI from storage settings and format it from
context menu, a user expects it goes back to storage settings after
formatting the storage.  However, previous we show Downloads as a result
of redirect from the previous USB storage.

The CL changes the logic for callback so that it finishes the activity
if DocumentsUI is opened for the specific root and the root was removed
while a user stays at the root.

Fixes: 28246076
Change-Id: I5548152fc27fd13bd9b75b3083bcfbdd9f93509e